Most fluorescent lighting systems have a fixed lumen output, except for phosphor degradation effects, i.e., the lumen generating arc is either off or on at a fixed level. However, a number of fluorescent lamps control techniques have, and are being, developed which permit proportional adjustment of the fluorescent lamp arc as a means to establish a more desirable level of lumen output. Savings in electrical energy accrue in such proportional lighting systems where the arc has been reduced from the otherwise "full on" level. Besides the economic implications, the ability to reduce lighting and still meet the requirements of different tasks improves lighting quality. For example, in order to achieve proper contrast and reduce reflections, the background lighting provided in an area surrounding a video display terminal (VDT) is generally lower than that required for high contrast work tasks like reading. Corridors require even lower lighting levels and, in general, to provide for uniform lighting in a building is wasteful of energy. Moreover, the ANSI lighting guidelines for different tasks and building areas vary over a range of at least ten to one, which makes adjustable lighting a highly desirable building lighting feature.
Proportional fluorescent lighting control systems generally use pulse width modulation of the fluorescent arc or variable frequency control of the arc, and/or some form of AC phase control of the fluorescent lamp arc discharge current and other parameters. Often, such systems require the adjustment of a control potentiometer to provide a varying reference or control signal used to establish the level of lamp lumen output. More advanced systems also employ a light sensing means for generating a light feedback signal which varies as a function of the surrounding ambient light. This light feedback signal, after conversion to a related electrical signal, and sometimes after summing with another reference signal, is used to proportionally reduce the average lamp arc current, and hence the light output from the lamps, when a daylight contribution is available. (In an open loop system, the summing operation referred to above may not be required.) Such systems are also designed to increase the average lamp arc current so as to maintain the ambient lighting level at the desired level originally set as the lamp phosphors wear and/or as the lamps and/or luminaire reflecting surfaces accumulate dirt. Without such an increase in the average arc current, an undesired lowering of the lumen output would occur. Presently, this problem is too often solved by the wasteful practice of over-lighting when lamps are new in order to ensure that there will be sufficient lighting when lamps and luminaires become degraded.
It has been shown that localized control of each luminaire is preferred because the contribution from daylight decreases with the distance between the luminaire and the window wall and daylight contributions in different areas may vary as a function of the azimuth angle of the sun, i.e., because of different shading effects produced as the sun moves across the horizon. With such localized control of individual luminaires, each fixture adjusts so that the sum of the daylight and the luminaire light meets the desired level. In contrast, in the case of larger area control of a plurality of luminaires, a standardized lumen output for all luminaires must be established in the area with the least daylight contribution. This results in over-lighting a large majority of the controlled area, i.e., that part of the area which receives a greater amount of daylight than the particular luminaire used to establish the lumen set point and adjustment is precluded for different work tasks or for variations in visual acuity between different workers whose work area is covered by that group of luminaires. Given that such local control is preferred, a simple economical means is necessary to set the lumen output in each local area at the recommended lumen level for the tasks to be performed in that area.
Localized control may be accomplished by utilizing a magnetic transformer ballast with an electronic controller, i.e., a controller such as disclosed in my U.S. Pat. No. 4,352,045, or a variable frequency or pulse width modulated electronic ballast or yet other means. In all cases, an optical pickup device is necessary to measure the localized light level. The light level measurement is converted into a corresponding electrical feedback signal. This signal is then used to control the lumen output of the luminaire. In this regard, the lumen output is increased if the ambient lumen level in the surrounding area decreases, e.g., as the daylight wanes. On the other hand, the lumen output is decreased if the ambient lumen level in the surrounding area increases, e.g., the amount of daylight increases.
Because the control unit, whether an electronic ballast or other controller, is generally mounted in a closed ballast compartment within the luminaire, the unit is not easily accessible for adjustment purposes. Hence, any light adjustment is best accomplished outside of the ballast compartment by maintenance personnel. In previous systems developed by me, this adjustment was made by attenuating the sensed light measurement by varying the distance from the light pickup lens to the optical-electrical transducer which converts the sensed light signal into a corresponding electrical signal. In this regard, in 1978 I developed a system employing an acrylic plastic lens as the light sensor, a fiber optic bundle and a photocell transducer wherein the distance between the lens and the fiber optic bundle was varied by manually adjusting the relative position of the fiber optic bundle in order to provide the desired set point. A system of this general type, i.e., one using a light collector and a fiber optic bundle, and one which, in practice, was adjusted as just described, is disclosed in my U.S. Pat. No. 4,234,820 (Widmayer).
A later development of my basic idea is disclosed in U.S. Pat. No. 4,383,288 (Hess, II et al) which is assigned to Conservolite, Inc., a then licensee of the assignee of U.S. Pat. No. 4,234,820 mentioned above. This patent discloses varying the output of a light collector device including a light sampler or gatherer (including a collecting lens) and a light receiver (including a fiber optic bundle) by, as in the prior art system referred to above, varying the distance between the light sampler and light receiver. This is accomplished in the light collector device of the Hess, II patent by mounting the light sampler (lens) with a threaded portion which is received in a threaded bore in the receiver so that by threading the light sampler into and out of the light receiver, the distance between the lens and the fiber optic bundle can be varied. The Hess, II patent provides that by threading the sampler further into the receiver, the amount of light transmitted to the illumination level control system is increased, thereby raising the illumination level in the controlled area, and that threading the sampler further out of the receiver produces the opposite effect. The light collector is illustrated in the patent as being secured to a specially shaped mounting bracket which is mounted on a T-bar of a suspended ceiling.
There are a number of disadvantages associated with the adjustable light collector of Hess, II. For example, the device requires either gripping the collecting lens directly with the fingers to provide the necessary threading in or out of the light sampler, or else requires the use of a special tool to perform this task, and it will be appreciated that both of these approaches have obvious shortcomings. Further, the mounting arrangement is cumbersome and the overall device is relatively complex given the purpose to be carried out.